Field of the Invention
The invention relates to a method for controlling an electromechanical actuator drive.
Internal combustion engines whose charge cycle valves are activated independently of the camshaft are known. In contrast to charge cycle valves that are activated by the camshaft, the charge cycle valves are actuated so as to open and close in dependence on a rotary position of the crankshaft. There is no fixed mechanical coupling to the crankshaft. Examples of electromechanical actuator drives for charge cycle valves are known from German Utility Model DE 297 12 502 U1 or Published, European Patent Application EP 0 724 067 A1. They have a position of rest that lies between a closed position and an open position and from which they can be deflected by electromagnets.
In order to open or close a charge cycle valve, the coil of the respective electromagnet is energized, the necessary current being greater in a capture phase than in a holding phase in which the charge cycle valve is held in a limit position.
Whereas there is no predefinition of the control times in the operating control unit of the internal combustion engine in the conventional, camshaft-activated valve driving mode, in electromechanically activated charge cycle valves corresponding control times must be calculated and predefined.
It is necessary to take into account here the fact that together with the actuator drive and its springs the charge cycle valve constitutes a spring-mass oscillator. Its natural frequency or resonant frequency determines the speed at which the valve can be moved between the limit positions.
As a result of the physical conditions a minimum actuating time from one limit position to the other limit position is predefined. It is known to take into account the minimum actuating time in the calculation of the control times.
From Published, Non-Prosecuted German Patent Application DE 195 26 681 A1, it is known to switch off the energization of the coil holding the actuator element in the limit position a certain time period before the time at which the actuator element is to be released from the limit position, because what is referred to as sticking of the actuator element in a limit position occurs as a result of mechanical and magnetic effects in the actuator drive. This is also mentioned in Published, Non-Prosecuted German Patent Applications DE 195 31 437 A1, DE 196 23 698 A1 and DE 195 18 056 A1.
It is accordingly an object of the invention to provide a method for controlling an electromechanical actuator drive that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which the effects of sticking are minimized.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for controlling an electromechanical actuator drive for driving an actuator element. The electromechanical actuator drive has at least one coil for holding the actuator element in a given position. The method includes the steps of switching-off an energization of the coil a given time period before a point in time at which the actuator element is to be released from the given position; and determining the given time period in dependence on a supply voltage of the electromechanical actuator drive and/or a coil current while the actuator element is held in the given position.
A precise examination has shown that the sticking depends on a decrease in the current in the coil, and this depends in turn on the supply voltage of the actuator drive and of the coil current level during the holding in the limit position. For this reason, in one variant of the invention, at least one of these variables is sensed and the time period is selected as a function thereof.
It has also become apparent that the mechanical sticking which is caused by adhesion effects in the actuator drive may he changed largely independently of the operating parameters and changed only slightly over the service life of the actuator drive. In contrast, the magnetic sticking caused by the decrease in the current in the coil depends on operating parameters of the actuator drive that can he sensed. In one preferred refinement of the method, the operating parameters are therefore sensed and used to determine a component time of the time period that is dependent on operating parameters. A constant variable, i.e. permanently stored variable, is used as a further component time, which, together with the above first component time, yields the time period. However, it can also be adapted by measuring the overall time period in a certain timing pattern.
With these methods, undesired control time fluctuations during the actuation of the actuator drive are avoided. In an internal combustion engine with electro-magnetically activated charge cycle valves, such control time fluctuations have a highly negative effect on exhaust gas emissions and smooth running, particularly when the inlet valves close.
In accordance with an added mode of the invention, there is the step of forming the given time period to be composed of two composite times including a first composite time and a second composite time, and only the first composite time is dependent on the coil current and/or the supply voltage.
In accordance with another mode of the invention, there is the step of selecting a constant value for the second composite time.
In accordance with further mode the invention, there is the step of adapting the second composite time in response to a determination of the given time period.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for controlling an electromechanical actuator drive, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.